1.10 - Transmission Between Cells & The NMJ

Question 1

What are the steps in chemical neurotransmission?

Answer 1

Neurotransmitter molecules
are synthesized and packaged
in to vesicles
An action potential arrives at
the terminal
Voltage-gated Ca2+ channel
open to allow Ca2+ influx in to
the terminal
Intracellular Ca2+ causes
synaptic vesicle fusion with the
presynaptic membrane
Transmitter diffuses across the
synaptic cleft and binds to specific membranes on the postsynaptic cell
Bound receptors cause opening of specific channels on the postsynaptic membrane
Neurotransmitter is removed from the cleft by various mechanisms

Question 2

How are neurotransmitters packaged in vesicles and how do they reach the terminal?

Answer 2

Peptide neurotransmitters are transported already packaged in vesicle with the peptide or its precursor and are transported from the cell body to the terminal
Non-peptide neurotransmitters are synthesised in the nerve terminal and transported in to the vesicle

Question 3

What is the importance of Ca2+ in neurotransmitter release

Answer 3

They provide the signal for fusion of synaptic vesicles and subsequent release of the neurotransmitter.

Question 4

Describe the opening of V-G Ca2+ channels during the action potential

Answer 4

During depolarization, Ca2+ voltage-gated channels open, but Ca2+ cannot enter the cell due to electrostatic charge, despite the chemical gradient favouring the entry of Ca2+
Ca2+ voltage gated ion channels are relatively slow to close, thus once the membrane potential repolarizes sufficiently, Ca2+ can enter the presynaptic terminal

Question 5

Describe the Synaptic Vesicle Cycle Hypothesis

Answer 5

Describes the fact that after exocytosis of the vesicle (and release of neurotransmitters) the synaptic vesicles are re-internalised via clathrin-mediated endocytosis. Synaptic vesicles are then reformed and refilled with neurotransmitter

Question 6

How is neurotransmission inactivated?

Answer 6

Very fast process.
Two main ways:
Re-uptake of the neurotransmitter (either the presynaptic terminal itself or support glial cells)
Cleavage/degradation of NT by enzymes.

Question 7

How is the action of ACh terminated in the NMJ

Answer 7

The the effect of Acetylcholinesterase (AChE). Breaks it down to Acetate + Choline

Question 8

Describe the production of ACh in the presynaptic terminal of the NMJ

Answer 8

ACh is produced from choline and the enzyme choline acetyltransferase (ChAT: Choline + AcetylCoA –> Acetylcholine)
ACh is packed in to vesicles via the ACh transporter